Leaf seals may be used to form a seal between two relatively rotating components in order to maintain a relatively high pressure on one side of the seal and a relatively low pressure on the opposite side of the seal. A leaf seal is arranged with a large number of typically rectangular leaves which are held at a defined angle (the “lay angle”) to the radial all the way round the seal circumference. The leaves give the seal a low stiffness, and the leaves are packed together such that the total leakage through the seal is reduced. Nonetheless, interleaf gaps do provide the seal with a porous aerodynamic working section. Such seals may be used, for example, in gas turbine engines.
FIG. 1 shows schematically a cut-away perspective view of a portion of a leaf seal 31 comprising a pack of leaves 32. FIG. 2 shows (a) a view along the axial direction of an arc segment of the pack to better show some of the leaves 32 edge-on, and (b) a plan view of a single leaf 32.
The leaves 32 each have a root portion 40 and a working portion 41, and have a width w in the axial direction and a thickness t. The leaves alternate with spacer elements 33 at their root portions 40, and are secured thereat to a backing ring 34 of a housing, which typically also comprises front 35a (high pressure side) and rear (low pressure side) 35b rigid cover plates. The working portions 41 of the leaves 32 present end edges 36 towards a surface 37 of a rotating component (for example a shaft) generally rotating in the direction depicted by arrowhead 38. The leaves 32, and in particular the end edges 36 of the leaves 32, act against the surface 37 in order to create a seal across the assembly 31. Each leaf 32 is sufficiently compliant in order to adjust with rotation of the surface 37, so that a good sealing effect is created. The spacers 33 ensure that flexibility is available to appropriately present the leaves 32 towards the surface 37 which, as illustrated, is generally with an inclined angle between them. The spacers 33 also help to form interleaf gaps 39 between adjacent working portions 41 of the leaves 32. A leakage flow through these gaps 39 is induced by the pressure differential across the seal. Leaf seals of an air-riding configuration are configured such that the end edges 36 of the leaves adjacent the rotating component are presented with a small air gap therebetween such that the leaves ride on the air leakage through that gap to inhibit premature contact wear of the leaf seal elements against the surface of the rotating component. It will be understood that the air gap should be as narrow as possible such that air leakage is reduced to the minimal level possible whilst creating the air-riding effect, so as to limit actual leakage across the seal. However, it has been found that it can be difficult to generate sufficient hydrodynamic lift between the leaf pack and the rotating component to provide a satisfactory air-riding cushion between the two.
It is normal for all of the leaves 32 in the pack to have an identical configuration. As illustrated most clearly in FIG. 1, the resulting leaf pack thus defines an inner bore 42 around the rotating component which is substantially parallel to the outer surface 37 of the rotating component. Such an arrangement can thus be considered to behave rather like a parallel bearing when in use, in the sense that it provides limited potential for the generation of hydrodynamic lift due to the fact that it provides two substantially parallel and relatively rotating surfaces (represented by the inner bore 42 and the facing surface 37 of the rotating component. The result is a leaf seal with poor hydrodynamic lift which is thus prone to wear at the tips of the leaves defining the inner bore around the rotating component and/or on the surface of the rotating component itself. This limits the useful life of the leaf seal.
It is a preferred object of the present invention to provide an improved leaf seal.